Anhydrous diluents for the isobutylene oxidation reaction to methacrolein and methacrolein oxidation to methacrylic acid

ABSTRACT

The processes for oxidation of isobutylene to methacrolein and the oxidation of isobutylene to methacrylic acid in two stages with methacrolein as an intermediate are improved by use of essentially inert essentially anhydrous diluent gases to replace steam in the reaction streams. In particular, the uses of essentially inert essentially anhydrous diluents which raise the composite heat capacity of the diluent gas mixture to at least about 6.5 calories/(gram-mole) (°C.) will improve selectivity to desired products and will reduce both the waste water load on the system and by-product formation.

This application is a continuation of prior U.S. application Ser. No.07/542,698 , filed Jun. 25, 1990 now abandoned which is a continuationin part of application Ser. No. 249,772, filed Sep. 26, 1988, nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for the manufacture of methacroleinor methacrylic acid from isobutylene. More specifically, it describes animproved process for producing methacrolein or producing methacrylicacid by the catalytic vapor phase oxidation of isobutylene in thepresence of essentially inert essentially anhydrous diluents.

2. Summary of the Prior Art

Generally, isobutylene in its gaseous phase is oxidized to methacroleinin the presence of molecular oxygen-containing gases and steam, whoseconcentration is often as high as about 35 volume percent of the totalfeed stream, by contact at elevated temperatures with solid metal oxidecatalysts. The methacrolein produced in this reaction stage can berecovered or can be directed without separation of the methacrolein to asecond reactor operating in series with the first reactor to oxidize themethacrolein to methacrylic acid.

In the prior art, steam has been used in the starting reactant gasmixture in order to avoid flammable gas mixtures and because it wasbelieved to be important to reaction selectivity, but there is noindication of composite heat capacity of the importance of the addeddiluent gas. For example, U.S. Pat. No. 4,147,885 states that in arelated process for oxidizing propylene to acrolein, it is wide practiceto incorporate steam to avoid burning the reactant gases and increaseselectivity to acrylic acid. Similarly, U.S. Pat. No. 3,475,488discloses that it is desirable to incorporate steam in the startingreactant gas since this increases conversion and selectivity whenemployed in the order of 1 to 60, and preferably 5 to 30, moles of steamper mole of propylene or propylene plus acrolein.

Other patents also describe steam as the preferred diluent. For example,U.S. Pat. No. 3,171,859 states that "addition of steam is obligatory . .. it acts not only as a diluent, but also favors the reaction in thatcombustion to carbon oxides is substantially reduced." Also, U.S. Pat.No. 4,267,386 reiterates the general understanding among those skilledin the art, that while inert diluents may be added to the reactionsystem, "water, in the form of steam is desirably present . . . inamounts of from 0.5 to 15, preferably 2 to 15, moles per mole ofunsaturated hydrocarbon (i.e., propylene or acrolein)." Again, with norecognition of composite heat capacity value of the diluent.

Many oxidation catalysts have been disclosed for producing methacroleinin high yield by oxidizing isobutylene. Predominantly, these arecatalysts containing mixed oxides of molybdenum, bismuth and iron withphosphorous or tungsten or antimony. Cobalt and/or nickel and alkalimetals are common promoters.

Catalysts which have been found to be advantageous for use in oxidizingmethacrolein to methacrylic acid generally contain mixed metal oxides.Such catalysts typically contain molybdenum, vanadium, tungsten,chromium, copper, niobium, tantalum and antimony.

It has heretofore been well recognized in the relevant catalyst art thatwater, in the form of steam, is an important component of the inert gasdiluent (see, e.g., U.S. Pat. No. 4,267,385, which states that it is"preferable to carry out the oxidation reaction in the presence ofsteam.") The ultimate object of the teachings in the literature citedabove is to obtain high performance catalysts which give highselectivities to methacrolein and methacrylic acid at high isobutyleneconversions. Other factors which influence the economic viability or theimproved performance of these processes are not considered in theseprior art techniques. For example, they do not adequately address theimpact on process variables of use of high isobutylene concentrations,how to avoid the danger of explosion, the impact of inert reactionprocess feeds on recovery and waste disposal, or maintaining highcatalyst performance over an extended catalyst life. These are allextremely important for commercial operation.

In commercial operation, it is of economic and ecological importance tominimize the presence of the steam which is fed to the reactors, sinceit passes through the system and becomes a burdensome waste water loadafter product recovery steps; nevertheless, to the knowledge of thepresent inventors, no commercial process has been successfully operatedbelow a steam: isobutylene mole ratio of about 1.5:1. Furthermore, it isextremely important to minimize by-products which are difficult toseparate from useful product or which carry a high economic penalty fordisposal. Process improvements which will provide high catalystperformance while simultaneously maximizing isobutylene feedstock usage,and improvements which may promote conditions for extended usefulcatalyst life are important for commercial operation. Equally importantis the ecological problem encountered when consideration has to be givento the disposal of millions of pounds of waste water, which will varyfrom about 0.5 to about 1.5 pounds of water per pound ofacrolein/acrylic acid produced, generated by a single multimillion poundcommercial facility for the production of methacrolein and/ormethacrylic acid. The typical commercial plant has an annual capacity offrom about 100 million to over 750 million pounds, thus giving anindication of the waste water problem. The prior art does not adequatelyaddress these issues.

U.S. Pat. No. 4,049,577 teaches an improved catalyst composition formaking acrolein. The authors mention that recycle gas comprised of thenoncondensable fraction of the product can be used in place of steam.They suggest that these recycled inerts are preferable to steam asdiluent since they allow higher conversions of propylene and thus enableone to obtain higher yields, and also reduce the water load on thesystem; however, the use of recycled inerts is stated as being madepossible by the characteristics of this particular catalyst composition.Nowhere does this patent suggest or teach that anhydrous diluents havinghereinafter defined composite heat capacity have an improved effect onselectivity or product mix, or are useful with other catalysts. Therelationships between various diluents and the heat capacity effects onselectivity are not suggested.

U.S. Pat. No. 3,801,634 teaches the use of inert solids mixed withactive catalyst in the first and second stage reactors used tomanufacture acrolein and acrylic acid. The authors indicate thatnoncondensable, second-stage effluent gases can be recycled to the firststage as inert diluting gas which can, at least partly, replace steam.The authors do not show any relationship between the inert anhydrousdiluent gases and improvements in product selectivity; or the desirableeffect of composite heat capacity value.

U.S. Pat. No. 4,031,135 presents a recycle process in whichnoncondensable gases, preferably and generally including steam, arerecycled to the first-stage reactor and also to the interstage(second-stage) reactor feed. There is no recognition of the benefits inusing anhydrous diluents having certain composite heat capacity valueswith respect to their effect on yield, conversion, by-productselectivity mix and waste water generation. The authors do, however,recognize an apparent improved acrylic acid efficiency, which theyattribute partly to the use of recycled off gas employed as the inertdiluent. In column 4, lines 13-15 the patent says the off-gas "has beensubstantially freed from condensable products, including water, andessentially consists of nitrogen and small amounts of" other namedcompounds. In column 6, lines 45 to 54 the general composition, involume percent of the off-gas is stated in broad and "especially" termsas being:

    ______________________________________                                                     broad  especially                                                ______________________________________                                        propylene      0-1.5    0.2-1                                                 oxygen         0-5      1-4                                                   CO/CO2         0-10     1-7                                                   acrolein       0-1      0.1-0.5                                               steam          0-10     0.5-5                                                 others         0-0.1    0.01-0.05                                             nitrogen       100-74   97.19-81.45                                           ______________________________________                                    

Further, in the only two examples presented in support of theirinvention the patentees specifically disclose the presence of 2% byvolume and 8.9% by volume of steam in the recycled off-gas clearly notan essentially anhydrous diluent off-gas stream. The patent clearlyteaches the recycle of nitrogen, in impure form, and steam in theoff-gas to the reactors. In the invention described in thisspecification the essentially inert diluent gas feed is an essentiallyanhydrous diluent gas composition, as hereinafter described. The gasfeed differs significantly from the teachings of U.S. Pat. No.4,031,135, it is essentially anhydrous. It is further to be noted thatthe patentees do not adequately establish improved efficiency to acrylicacid production, and that they do not recognize or disclose the effectof composite heat capacity of the diluent on product selectivities.

U.S. Pat. No. 4,365,087 refers to the recycling of dewatered residuegas, containing both inert and reactive gases, to increase theconcentration of acrylic acid recovered. However, the authors not onlyconsider this procedure unsatisfactory since the composition of theresidue gas fluctuates but have no recognition of the concept ofcomposite heat capacity of the diluent and its effect on the process.

U.S. Pat. No. 4,442,308 teaches the use of inert gases as diluent in theacrolein process; however, it specifies their use for a particularsupported first-stage acrolein catalyst. Most common commercialcatalysts for propylene oxidation to acrolein are neat (unsupported) anddo not follow this patent's prescribed preparation. This patent alsoclaims that 0.5 to 7 mole % steam is beneficial and its use isrecommended. Nowhere in this patent do the authors teach the advantageof anhydrous diluents on product mix nor do they mention composite heatcapacity or flowing heat capacity as major variables in controllingproduct selectivity to advantage and formation of undesired productstreams.

U.S. Pat. No. 4,456,006 teaches a catalyst preparation for thepropylene-to-acrolein reaction. It shows that nitrogen diluent presentsan improvement over steam diluent when used with this catalyst. It doesnot recognize or disclose the composite heat capacity effect of diluenton product selectivity, nor does it show by-product and waste waterreductions when using anhydrous diluents.

U.S. Pat. No. 3,717,675 describes a process for recovery of acrylic acidwhere acrolein is expelled from the aqueous acid collected and returnedto the reactors to increase subsequent yields of acrylic acid. Thispatent mentions the use of inert diluents such as carbon oxides andnitrogen, but does nothing to demonstrate their importance. In fact, itstates that it is necessary to add steam to the reaction in order toincrease selectivity. This addition of steam, however, only serves toaggravate the waste water disposal problem.

UK Patent 2,068,947 teaches a process for producing methacrolein andmethacrylic acid whereby inert anhydrous diluent gases are used, alsocombined with water vapor, to produce a product with a reduced quantityof condensables compared to the typical steam diluent process. Theauthors fail to recognize the relationship between anhydrous diluentsand acetic acid reduction, and they do not address composite heatcapacity of the diluent or selectivity improvements resultant from useof various anhydrous diluents.

U.S. Pat. No. 4,147,885 describes a recycle process in which steam is anessential ingredient. The object of the patented invention is to recyclesteam to the reactors. This is contrary to the techniques of the instantinvention, since it has now been found that the reduction or absence ofadded steam to the reactors is beneficial.

U.S. Pat. No. 4,618,709 presents an attempt to remedy, or at leastalleviate, the waste water problem common to the existing catalyticoxidation processes for producing methacrolein and methacrylic acidfrom, e.g., isobutylene. This is accomplished in this patent byevaporating the waste water solution and subjecting the waste watervapor to combustion with molecular oxygen, whereby the amount of liquidwaste water discharged is reduced. As can be seen, this is an expensiveprocedure since it involves two additional costly steps. In discussingthe isobutylene oxidation process the patent refers to the well knownand common procedure of carrying out the oxidation in the coexistence ofan inert gas for dilution (column 1, lines 22-35 and 49-53) to controltemperature and prevent explosion. The patent then mentions "nitrogen,water vapor, exhaust gases, etc." as examples of inert gases added andfurther states water vapor as being the most frequently employed, in anamount as great as 10 to 50 moles of water, or water vapor, added permole of methacrolein or its precursor (column 1, lines 54-58). Thesefigures clearly evidence the intentional addition of significantquantities of water to the oxidation reaction, water that becomescontaminated with reactants and products of the reaction and mustsubsequently be disposed of in an ecologically accepted mode. Thereference nowhere suggests or discloses the importance of using an inertgas having a composite heat capacity of certain value for dilution. Nordoes the reference recognize or suggest the importance of avoiding theintentional addition of supplemental quantities of water to the reactionsystem.

UK Patent Specification 939,713 disclosed one of the earliest catalyticprocesses for preparing unsaturated monocarboxylic acids from olefins.In these early processes yields of acrolein and acrylic acid were low,as evidenced by the figures presented in the examples that show lowconversion, and overall yields of less than about 75% from propylenecharged. By comparison, today's processes operate at exceptionally highyields, and conversions that typically approach 95% to 98%. On page 2,lines 4 to 19, this UK Specification refers to the starting materialsand indicates they need not be in a pure state and may containquantities of paraffinic hydrocarbons, such as propane or butanes. It isto be noted it is present in the starting olefin reactant as an impurityand it is not intentionally additionally introduced into the reactant asa diluent. It is also noted that its quantity is nowhere clarified andthat its function is described as an entraining agent; there is norecognition of its possible use as a medium for heat removal, nor doesthe reference state or suggest that the hydrocarbons have any effect onmaintaining the desired temperature range. Though the reference statesthe process can be operated in the absence of water (page 2, lines92-94), this statement is both immediately preceded and followed by thestatement that water is preferably used in quantities of from 1 to 10moles of water, preferably 3 to 7 moles for each mole of olefininitially introduced into the first reaction zone and all of theexamples use significant amounts of water. The reference does state thereaction can be carried out in the absence of water but it nowhereindicates the water must be replaced. It merely states do not add thewater. Any attempt to carry out this reaction without any added diluentwould be catastrophic. In Examples I and II, 4.2 moles of water wereintentionally added per mole of propylene, in Example III, 5.8 moles ofwater were intentionally added per mole of propylene and in Examples IVand V, 12 moles of water were added per mole of propylene. Thus, watercomprised the majority of the bulk of the materials introduced in allexamples. Further, nowhere in this UK Specification is there any mentionof the use of any other coolant or temperature control medium. Nor isthere any suggestion or recognition of the importance of the compositeheat capacity of the gas diluent and its effect on conversion and yield.

None of the prior art suggests or recognizes the use of various inertanhydrous diluents in specific proportions so as to have the hereinafterdefined composite heat capacity that will favorably affect the productmix obtained when using any of the commonly used catalysts.

As has been indicated above, the basic two-stage process for oxidizingisobutylene to methacrylic acid via methacrolein is well known and hasbeen extensively described in the literature. In connection with therelated acrylic acid process, it is also known that wet, overhead gases(noncondensables) from the acrylic acid scrubber can be recycled to thefirst reactor stage. By this recycling of unreacted propylene andacrolein, it is predictable that an improvement in overall yield isobtained in any chemical reaction. By use of such a recycle stream, itis also possible to provide a supplemental means of controlling thesteam content to the first-stage reactor, as is taught in U.S. Pat. No.4,147,885. In the process of that patent, the steam content of thefirst-stage feed is required to be 4 to 30% by volume, with all thesteam, except that in the starting reactant gas mixture, being providedby the recycle stream. As discussed above, however, the presence of evenas little as 4% steam is disadvantageous. This finding has not beenaddressed, nor even identified, by the prior art.

As has also been indicated above, nowhere in the prior art is there anydisclosure or suggestion of the important role exerted on the process bythe composite heat capacity of the diluent gas mixture and of theunexpected and unpredictable effect exerted by the composite heatcapacity of said mixture on yield, conversion, by-product formation andwaste water generation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate the application of recycle streams tomethacrolein and methacrylic acid processes.

SUMMARY OF THE INVENTION

The present invention embraces two separate but related concepts,namely, the reduction or elimination of steam to reduce the water loadon the process, and the improvement of selectivity to maximize theoutput of desired products. Both of these results are achieved byelimination of the steam diluent intentionally added in prior artprocesses and use of an essentially anhydrous gaseous diluent(optionally containing a minimal amount of steam which is generated fromtrace impurities of water that may be present in the reactants anddiluents initially charged to the reactor) having a composite heatcapacity in a selected range as hereinafter defined.

In a preferred embodiment, a portion of the non-condensable essentiallyanhydrous gases from the process, e.g., the overhead stream from theacid scrubber, is recycled back to the first-stage reactor feed stream.In addition, most preferred would be the use of enriched or pure oxygenin the two stages to replace the use of air.

It will be understood that the process of this invention can be appliednot only to a combined isobutylene methacrolein-methacrylic acidprocess, but also to a separate methacrolein-methacrylic acid process,or to the methacrolein-methacrylic acid leg of anisobutylene-methacrylic acid process. Thus, a portion of the stream fromthe first-stage isobutylene-methacrolein reactor can be sent to amethacrolein recovery process, from which some or all of thenon-condensable overhead gases from the methacrolein scrubber system canbe recycled as diluent to the first and/or second-stage of theisobutylene methacrylic acid process.

DESCRIPTION OF THE INVENTION

For purposes of this invention, the term "isobutylene" is intended toinclude as well other compounds which can be converted in situ toisobutylene, e.g., t-butyl alcohol, t-butyl acetate, methyl t-butylether, and the like, and/or which respond to the same catalysts as doesisobutylene and proceed through the same intermediates (i.e.,isobutylene and methacrolein).

According to the invention, it has been discovered that essentiallyanhydrous diluent gases having high heat capacity can be used to reduceor essentially completely replace steam in the isobutylene oxidationreaction to efficiently produce methacrolein and methacrylic acid. (Forpurposes of this invention, a diluent is any gas which does not react inthe reaction stage in which it exists.) Furthermore, when replacing thesteam diluent with essentially anhydrous diluents, major by-products,e.g., acetaldehyde and acetic acid, are significantly reduced. Thisreduction in by-products is especially important since acetaldehydeforms acetic acid over the methacrolein-to-methacrylic acid reactionstep, and acetic acid is a significant waste product. To make saleablequality methacrylic acid, considerable energy is required to morecompletely remove the acetic acid. Waste disposal costs for disposing ofacetic acid are high. The present invention provides a means forreducing acetic acid disposal costs, decreasing separation costs forboth methacrolein recovery and methacrylic acid recovery, and enablesexisting equipment to enact a better separation, thus saving methacrylicacid recovery losses and providing potential for higher quality refinedproduct.

Another key discovery of this invention is that by increasing theflowing heat capacity of the reactant gas mixture, the yield of usefulproducts can be increased significantly. The flowing heat capacity isincreased by the introduction of an essentially anhydrous diluent havinga relatively high composite heat capacity (as defined herein),comprising one or more essentially inert essentially anhydrous gaseswith relatively high molar heat capacities. Flowing heat capacity is thecomposite heat capacity of the essentially anhydrous diluent, plus theheat capacity of the reactants, i.e., the flowing heat capacity is thecomposite heat capacity of the total gas stream. However, flowing heatcapacity does not change appreciably as a result of reaction, sincevarious reaction products have a higher heat capacity than that of thereactants, and some have a lower heat capacity. In general, the flowingheat capacity will not be expected to typically change by more thanabout one heat capacity unit as a result of reactions. Thus, thecomposite heat capacity of the diluent is a dominant variable forprocess control purposes.

As the flowing heat capacity of the reaction feed gas mixture isincreased, yield to methacrolein, and methacrolein plus methacrylic acidincreases, and the flammable gas range is reduced, enabling higherproductivity operations. Simultaneously, the peak temperature in thecatalyst bed due to the exothermic heat of reaction is lessened and theheat of reaction that is released is absorbed more efficiently in thebulk gas stream. This, in turn, should increase catalyst life bydecreasing thermal stresses within the catalyst pellets' structure,reducing potential carbon build-up within catalyst pores, and byreducing pressure drop, since there will be lower volumetric flow ofreactant gas feeds necessary to meet a given production level.

The invention is advantageous for recycling diluent gases and unreactedisobutylene back to the reactors. The resulting low-steam-containingproduct streams provide an ample source of noncondensable diluent, sothat separation of useful product is simplified. This is particularlyadvantageous for methacrolein recovery, since methacrolein, which ismore volatile than water, can be effectively separated from thereaction-produced water without loss of diluent. By using essentiallyanhydrous diluents with higher volatilities compared to methacrolein,the present invention permits operation on a methacrolein recoverysystem with recycle of diluent, unreacted isobutylene, and unrecoveredmethacrolein back to the reactor for further efficiency gains and costreductions. Such a system using steam diluent is not possible whenadapting prior art methacrolein recovery equipment and techniques. Italso enables implementation of recycle processes in which componentssuch as acetic acid and acrylic acid and other minor, heavy by-productsare excluded from the recycle stream. This is significant since theacids and heavy by-products are suspected of adversely affectingcatalyst life, and furthermore, it aids in minimizing recycle handlingproblems, such as compressor corrosion.

The composition of the process feeds must be comprised so that flammablegas mixtures are not formed. According to this invention, the startingreactant gas mixture to the first-stage reactor typically contains up toabout 16 g-moles per hr. of isobutylene, preferably up to about 8g-moles per hr. of isobutylene, per liter of first-stage catalyst; about1.1 to about 2.2 moles of molecular oxygen per mole of isobutylene, andan essentially inert diluent gas having a composite heat capacity of atleast about 6.5 calories/gram-mole (°C.) which comprises about 40 toabout 94% by volume of the feed stream. The oxygen source can be air,oxygen-enriched air, essentially pure oxygen or a mixture of oxygen andessentially inert anhydrous gases. As used in this specification by theterm essentially inert anhydrous diluent gas (or variant thereof) ismeant the inert gas stream of one or more gases introduced into areactor to which additional water in any form has not intentionally beenadded before the inert gas is introduced into the reactor but whichinert gas stream may contain trace impurities of water, or which watermay have been introduced into the reactor as a trace impurity present inthe oxygen feed, or formed during the reaction. It is desirable that themole ratio of composite diluent to isobutylene be in the range of about2 to about 32. The essentially anhydrous diluent gas typically comprisesa mixture of nitrogen, carbon dioxide, methane, ethane, propane andbutanes; however, any other essentially anhydrous inert gas can beincluded. Some other useful inert gases include helium, argon, saturatedhydrocarbon gases, N₂ O, and carbon monoxide. When water is present as atrace impurity in any of the materials introduced into the reactors, atthe elevated temperature required for these reactions the water isimmediately converted to steam. The materials used should preferably befree of water, but in those instances in which water may be present asan impurity, the total amount thereof in all materials added should beno more than about 0.4 mole per mole of isobutylene, preferably 0 toabout 0.3 mole per mole of isobutylene, and most preferably zero. Theinert diluent should be of sufficient quantity to avoid flammablemixtures when combined with the isobutylene and molecular oxygen. Air oroxygen stream or pure oxygen can be used as the molecular oxygen source.Of course, if air is used, the contained nitrogen acts as a supplementaldiluent. In the process of this invention the intentional introductionof extraneous steam to a reactor is not contemplated.

For each inert essentially anhydrous diluent gas mixture there is arelationship which can be determined by experiment and which describesthe limiting compositions of oxygen, isobutylene, and diluent for whichflammable mixtures exist. Most commercial applications are mostdesirably operated in a "fuel-rich" mode, whereby the oxygen content isthe limiting factor from a flammability standpoint. The isobutyleneconcentrations will be determined by catalyst performance and bycommercial cost effectiveness factors.

It is a distinct advantage of this invention that, since diluent gasmixtures with high composite heat capacities have a tendency to broadenthe operable range due to shrinkage of the flammable gas envelope, highisobutylene concentrations are possible. It is theorized that firststage isobutylene feed concentrations as high as about 30 mole % will beachievable using the method of this invention.

Typically approximate ranges for feed compositions are defined based onthe generalized operating constraints discussed above. First-stage feedsin the following ranges are typically particularly useful:

Isobutylene: up to about 16 g-mole per hr. per liter of first-stagecatalyst, preferably up to about 8 g-mole per hr. per liter offirst-stage catalyst;

Oxygen: 1.1 to 2.2:1 O₂ /C₄ H₈ ratio, such that there is up to about33.6 g-mole O₂ per hr. per liter of first-stage catalyst, preferably upto about 21 g-mole O₂ per hr. liter of first-stage catalyst;

Diluent: About 2.0 to 32:1 diluent/C₄ H₈ ratio, preferably 3.5 to 12diluent/C₄ H₈ ratio. Nevertheless one can use amounts slightly below orabove the above ratios, e.g., one can go as low as about 0.5:1 and ashigh as about 33:1.

The process of the invention is particularly advantageous in that it isnot dependent upon any particular catalyst, as is much of the prior art,and will provide its benefits for any catalyst of choice. Anymolybdenum, iron-based mixed metal oxide oxidation catalyst, such asthose disclosed in U.S. Pat. Nos. 3,825,600; 3,649,930; 4,339,355;4,267,385; and 4,306,090; (catalysts relevant to the first stage arealso shown in U.S. Pat. Nos. 4,012,449 and 4,354,044) can be used in theisobutylene-to-methacrolein oxidation reactor. Any Mo, P-based mixedmetal oxide oxidation catalyst, such as described in U.S. Pat. Nos.4,419,270; 4,444,907; and 4,051,179 can be used effectively in thesecond stage (i.e., the methacrolein oxidation to methacrylic acidreaction).

The general reaction conditions are not narrowly critical, and are thoseknown to the art. The first-stage reaction operates at temperatures of250° C. to about 450° C., although temperatures of about 300° C. toabout 400° C. are preferred. The second-stage reaction requirestemperatures of about 200° C. to about 450° C., with a preferred rangeof about 250° C. to about 375° C.

Operating pressures of about 1 to about 4 atmospheres are typical,although this process improvement will apply for all operatingpressures, whether subatmospheric, atmospheric, or superatmospheric.Preferred commercial modes of operation will minimize pressures, butpressures are typically held in the 2 to 3 atmosphere range due tosystem pressure-drop constraints.

Flow rates can be varied from about 0.5 to about 15 seconds contacttime; however, typical commercial flow provides about 1.5 to about 4seconds contact time. Contact times of about 1.7 to about 3 seconds arepreferred.

As indicated above, selection of proper heat capacity of the essentiallyinert anhydrous diluent gas or gases is critical to the properperformance of the invention. Since the essentially inert essentiallyanhydrous diluent gas stream may comprise a mixture of severalindividual gases, it is convenient to refer to a composite heat capacityfor the total stream. The term "composite heat capacity," as usedherein, means the sum of the products of the volumetric fraction of eachgas in the diluent gas mixture and its heat capacity. (Heat capacity, asreferred to herein, is the ideal gas heat capacity determined at 330° C.for purposes of the composite heat capacity definition.) The compositeheat capacity for the essentially inert anhydrous diluent gas going tothe first stage reactor should be at least about 6.5 calories/gram-mole(°C.). Below this value, the product selectivity benefits of thisinvention are minimal. There is no known upper limit on composite heatcapacity; however, it is theorized that above a value of about 40 theremay be an unrecoverable heat loss through absorption of reaction heatinto the process stream, which would result in an economic penalty. Inaddition, there could be a problem with increased after-burning at theexit of the first-stage reactor. It is preferred that the composite heatcapacity be maintained from about 8 to about 30, and most preferablyabout 10 to about 20. Assume the presence of four gases in the inertdiluent gas stream, A, B, C, D, in volumetric presence of 20% A, 40% B,30% C, 10% D. Assuming heat capacity in degrees centigrade of w cal/gram-mole for gas A, x cal/gram-mole for gas B, y cal/gram-mole for gasC and z cal/gram-mole for gas D. Then the "composite heat capacity"(CHC) of the inert anhydrous diluent gas stream is expressed by theequation:

    CHC=(0.20)(w)+(0.40)(x)+(0.30)(y)+(0.10)(z)

The sum of these should be at least about 6.5 calories/gram-mole (°C.)as indicated above.

The flowing heat capacity of the second-stage reactor feed gases isdetermined predominantly by the choice of essentially inert anhydrousdiluent gas fed to the first-stage reactor. The first-stage product mixhas only a minor influence on the second-stage feed flowing heatcapacity, since the products typically account for only about ten totwenty percent of the total stream volume. For example, a typicaloperation with 6% isobutylene and 13% oxygen produces water plusmethacrolein, methacrylic acid, acrolein, acrylic acid, acetaldehyde,acetic acid, and carbon oxides. The average heat capacity of the feedisobutylene and oxygen is very nearly the same as the average heatcapacity of the resulting products (approximately 0.65 cal/g-mole (0°C.) more for products versus reactants).

The essentially anhydrous diluent gas of this invention introduced intothe reactor can be a single gas or a multi-component mixture of gases,provided that certain criteria are observed. Each gas must beessentially inert to the oxidation reactions of the process, and eachgas must be non-condensable under typical operating conditions andreadily separable from the reaction products.

Since each plant installation will have specific constraints that affectthe energy usage for the entire plant, attention must be paid to theimpact on the plant energy balance for use of particular diluents whichalter the current heat recovery schemes. For example, a high heatcapacity diluent will retain more of the heat evolved through reaction,whereas now the bath temperature is relied on more to remove and recoverthe heat of reaction. High heat capacity diluents will require moreattention to recovering heat after the reaction. Furthermore, if processoff-gas is disposed of via combustion, the recovery of heat will beaffected by a major change in diluents.

In addition, one should avoid catalyst poisons, e.g., sulfur dioxide,and gases that react to form unwanted by-products, as would unsaturatedhydrocarbon compounds (e.g., propylene); or NH₃, which producesmethacrylonitrile.

It is another advantage of this invention that the steam componenttypically contained in the feed to the first stage can be minimized,even eliminated. While there is controversy among those skilled in theart as to the precise function of steam, e.g., whether it is truly aninert diluent or whether it somehow participates in the oxidation ofisobutylene and methacrolein, it is accepted practice in the art as iscurrently performed that a significant concentration of steam isrequired in order to successfully operate the first-stage andsecond-stage reactions. Contrary to this holding of the art, it was asurprising, unexpected and unpredictable discovery of the presentinvention that intentional addition of steam is desirably eliminatedentirely. This is accomplished by substituting for the steam theessentially inert essentially anhydrous gas diluent of selectedcomposite heat capacity as described in this invention. Accordingly, ithas been found that the steam content of the feed gas can be essentiallyzero. While not preferred, the steam content of the feed gas can rangeas high as about 3% by volume of the feed gas when the materialscomprising the feed gas have not previously been treated to removewater. However, it is preferred that the steam content resulting fromwater as an impurity in the feed stream be kept below about 2% morepreferably below about 1% by volume and most preferably zero. Sincesteam is not intentionally added to the reactors this waste waterdisposal problem is significantly reduced.

While not an absolute requirement of this invention, it is highlypreferred that the essentially inert anhydrous diluent gas used be, atleast in part, an essentially anhydrous recycle stream from within theprocess. Preferably, this will comprise a portion of thenon-condensable, overhead gas mixture from the methacrolein ormethacrylic acid recovery scrubber train which removes water andmethacrylic acid from the product mixture. In particular, the use oflow-boiling, anhydrous diluents makes recycle from a methacroleinrecovery process possible. Besides providing a beneficial system whichallows recovery of current methacrolein separation efficiency losses andreuse of unconverted isobutylene, recycling process gases from amethacrolein recovery process is desirable and advantageous to recyclingprocess gases from a methacrylic acid recovery process as described inthe prior art. Generally, in the process of this invention additionalhigh heat capacity inert diluent is added to the recycle stream.

In order to minimize the water vapor carried overhead from thescrubbers, they should be operated within the ranges of conditions shownin Table A.

                                      TABLE A                                     __________________________________________________________________________                              METHACRYLIC ACID SCRUBBER:                                                    (Methacrolein Recovery System                                                                    METHACROLEIN SCRUBBER:                                     Methacrylic Acid Recovery System                                                                 (Methacrolein Recovery                                                        System)                          __________________________________________________________________________    BASE TEMP. (°C.)   <95, preferably: 80                                                                              <45, pref.: 15 to 35             HEAD TEMP. (°C.)   <80, pref.: <70    <40, pref.: 10 to 30                                       most pref.: <60                                     PRESSURE (ATM)            <3, pref. 1 to 2   <3, pref.: 1 to 2                SCRUBBING MEDIUM FLOW (Volume)                                                                          <1:1                                                BOTTOM PRODUCT STREAM FLOW (Volume)                                                                     pref.: <1:2                                         SCRUBBING MEDIUM FLOW (Weight)               <80:1                            ACROLEIN BOTTOMS FLOW (Weight)               pref.: <30:1                     __________________________________________________________________________

Under most operating conditions, it will be necessary to take off apurge stream, the size and location of which will be determined by thespecific process being used. If pure oxygen is used as the source ofoxygen, the purge can be relatively small. If air is used as the oxygensource, there will be a build-up of inerts, e.g., nitrogen, so that asubstantial purge will be required, and will be controlled to maintainthe desired composite heat capacity. The use of pure oxygen (i.e.,oxygen not burdened with substantial concentrations of inert gases)permits maximization of diluents with a heat capacity higher than thatof nitrogen. This permits minimization of the purge, which in turn,makes feasible the use of high heat capacity gases, such as propane orbutane, which might be too expensive for use as inert diluents if theywere to be substantially lost through purging.

EXAMPLES

The examples which follow illustrate the invention, but are not intendedto limit it in any way. In these examples, all concentrations are inmole percent.

Comparative Experiment A

A pilot plant experimental set-up consists of two single tubularreaction vessels of typical commercial reactor tube dimensions. Thefirst reactor tube contains a commercial catalyst comprising molybdenum,bismuth, iron and several promoter metals typical of first-stagecatalyst, as described above. The second reactor tube is filled with acommercial second-stage catalyst, similar to those described previously,and is connected in series with the first. The gaseous reaction productsare sampled and separated into condensable and noncondensable portions.Each phase sample is measured and analyzed by gas chromatograph. Theresultant measurements are used to calculate reaction yields andisobutylene conversions. These sampling procedures are accomplished forboth first-stage product and second-stage product, so that processperformance for methacrolein production and process performance formethacrylic acid production are both determined. A jacket surroundingeach tube is filled with a heat transfer fluid which circulates toremove heat of reaction. Thermocouple and sample taps are provided alongthe length of each reactor and at the bottom of each reactor. Gas feedsare metered into the first reactor using mass flowmeters. Thefirst-stage effluent is then conducted directly into the second-stagereactor. The condensable portion of the second-stage effluent isrecovered as a liquid tails streams from a water-based scrubber.Non-condensable gases are conducted out the top of the scrubber andcould be returned, if desired, to the reactors to supply additionaldiluent gas. The system outlet pressure is controlled at 7 psig in orderto control reactor feed pressures on the system. Isobutylene feedconcentration is set at 6.0% and air feed concentration is set at 60.2%.The additional (to nitrogen in the air feed) feed gas diluent contains2.6% nitrogen and 30% steam. (Also present is about 0.2% inertimpurities in the isobutylene.) The system outlet pressure is set at 7psig, and reactor temperature is set to 350° C.

A typical isobutylene conversion of 98% with 85% selectivity tomethacrolein plus methacrylic acid is achieved.

EXAMPLE 1

Experiment 1 is repeated, but with 32.6% nitrogen and 0% steam as thediluent gas. Temperature is adjusted to give a first-stage isobutyleneconversion of 98%.

A reduction of about 50% in acetaldehyde and acetic acid byproducts isobservable. Methacrolein plus methacrylic acid selectivity is increasedby about 1%.

EXAMPLE 2

The conditions of Example 1 are repeated with methane instead ofnitrogen in the diluent gas, the remainder being steam.

Acetaldehyde and acetic acid byproduct selectivities are reduced byabout 50%. Methacrolein plus methacrylic acid selectivity increases byabout 3%.

The terms "conversion," "yield," "selectivity," "space velocity," and"contract time" are defined as follows: ##EQU1##

Recycle Applications

Recycling of process streams is well known in the chemical process arts,and is usually implemented to improve reaction efficiencies and processeconomics. More specifically, recycling of product or a portion of aproduct stream enables efficient use of feed material not reacted in asingle pass or reuse of feed material which is costly to make up in thereactor feed stream. Use of essentially anhydrous diluents has aparticularly advantageous effect on the operability of recycle. Itenables using a recycle stream which has less acid, thus increasingcompressor operability. Furthermore, prior art recycle processes requiremore elaborate sampling mechanisms in order to reliably measure recycledoxygen concentrations. The control of oxygen is essential to safeoperation of these recycle processes due to concerns over flammable gasmixtures. The essentially anhydrous streams of this invention, howeverprovide for reliable and accurate monitoring of oxygen, therebyincreasing the recycle process reliability and operability as well assafety.

Furthermore, an essentially anhydrous diluent process allows simple,efficient recovery and recycle of methacrolein in a methacroleinproduction unit.

We claim:
 1. In a process for producing methacrolein by the catalyticoxidation of isobutylene and a process for producing methacrolein andmethacrylic acid by a two-stage catalytic oxidation of isobutylene,wherein the first stage reaction produces primarily methacrolein and thesecond stage reaction produces primarily methacrylic acid by oxidationof methacrolein, said process utilizing one or more recycle streams toeither or both stages, both stages operating on feed streams containingoxygen and added inert diluent gas, the improvement comprising utilizingone or more essentially inert essentially anhydrous diluent gases freeof any intentionally added water in a mole ratio of about 2.0 to about32.0 moles of diluent per mole of isobutylene as the essentially inertessentially anhydrous diluent feed added to the first stage, said addedessentially inert essentially anhydrous gas feed having a composite heatcapacity from about 8-30 calories/gram-mole °C. and an oxygen-containingstream containing from about 1.1 to about 2.2 moles of molecular oxygenper mole of propylene and utilizing one or more essentially inertessentially anhydrous diluent gases as the inert gas feed which is freeof any intentionally added water to the second stage, said addedessentially inert essentially anhydrous diluent comprising one or moreinert gases having a composite heat capacity from about 8-30calories/gram-mole °C.
 2. A process as claimed in claim 1 wherein thecomposite heat capacity of the essentially inert diluent gas to thefirst stage is about 10to about
 20. 3. A process as claimed in claim 1wherein the total amount of water present in said essentially inertessentially anhydrous diluent gas feed to the first-stage is less thanabout 0.4 mole per mole of isobutylene.
 4. A process as claimed in claim3 wherein the steam content of the essentially inert essentiallyanhydrous diluent gas to the first stage is less than about 0.3 mole permole of isobutylene.
 5. A process as claimed in claim 1 wherein theoxygen is from a pure oxygen source.
 6. A process as claimed in claim 1wherein the essentially inert essentially anhydrous diluent gascomprises a recycled process stream from a methacrolein recoveryoperation.
 7. A process as claimed in claim 1 wherein the essentiallyinert essentially anhydrous diluent gas comprises a recycled processstream from a methacrylic acid recovery operation.
 8. A process asclaimed in claim 1 wherein the methacrolein produced in the first-stagereactor is separated and recovered.
 9. A process as claimed in claim 1wherein the methacrylic acid produced in the second-stage reactor isseparated and recovered.
 10. The process of claim 1 wherein the moleratio of diluent to isobutylene is from about 3.5 to about 12 to
 1. 11.The process of claim 1 wherein the mole ratio of diluent to isobutyleneis about 11.5 to 1 and the mole ratio of oxygen to isobutylene is about1.8 to
 1. 12. The process of claim 1 wherein the diluent gases arenitrogen and methane.